Reason #1
Wait it does what❓
Your voting power can not go over 100%. It waits till voting power is about to go to waste, then prevents that waste to support the group.
If your voting power reaches close to 100% it will vote on your behalf for other group members.
Reason #2
But how❓
Your position for a vote is determined by how many votes has been produced on your behalf. You will be refunded for your votes, keep posting, keep letting it vote.
You'll get the maximum out, when you're behind, you will get more votes. When you're overpaid, it will spread to other users a bit first.
So if you don't post for a while, you can post more often to catch up.
You just join, and post, that's all you do, the more you let your voting idle, the more you'll get from the group.